Methods and apparatus for ultrasound imaging

ABSTRACT

A first ultrasound pulse is applied to biological tissue to create shear waves in the biological tissue, a focused ultrasound pulse is transmitted into the biological tissue, one or more ultrasound signals is received from the biological tissue, and shear waves are detected in the biological tissue based on the received one or more ultrasound signals. At least one propagation property associated with the detected shear waves is determined, and the determined at least one propagation property is displayed.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 61/321,005, filed on Apr. 5, 2010 and entitled“Method and Apparatus for Ultrasound Imaging”, U.S. Provisional PatentApplication Ser. No. 61/321,341, filed on Apr. 6, 2010 and entitled“Method and Apparatus for Ultrasound Imaging” and U.S. ProvisionalPatent Application Ser. No. 61/350,585, filed on Jun. 2, 2010 andentitled “Method and Apparatus for Ultrasound Imaging”, the contents ofwhich are incorporated herein by reference for all purposes.

BACKGROUND

Systems and methods described herein generally relate to the field ofultrasound imaging. More specifically, embodiments described belowrelate to methods and systems for measuring shear wave velocity intissue.

Pathological conditions may result in soft tissue which is stiffer thanwould be present under physiological conditions. Physicians thereforeuse palpation to locate stiff tissue within a body and thereby identifypathological conditions. For example, breast cancers are known to begenerally harder than healthy breast tissue and may be detected as ahard lump through palpation.

The propagation velocity of shear waves in tissue is related to thestiffness (Young's modulus or shear modulus) of tissue by the followingequation,

E=3ρ·c ²  (1)

where

c is the propagation velocity of shear wave, E is Young's modulus, and ρis the tissue density. Therefore, cancers or other pathologicalconditions may be detected in tissue by measuring the propagationvelocity of shear waves passing through the tissue.

A shear wave may be created within tissue by applying a strongultrasound pulse to the tissue. The ultrasound pulse may exhibit a highamplitude and a long duration (e.g., on the order of 100 microseconds).The ultrasound pulse generates an acoustic radiation force which pushesthe tissue, thereby causing layers of tissue to slide along thedirection of the ultrasound pulse. These sliding (shear) movements oftissue may be considered shear waves, which are of low frequencies(e.g., from 10 to 500 Hz) and may propagate in a direction perpendicularto the direction of the ultrasound pulse. The ultrasound pulse maypropagate at a speed of 1540 m/s in tissue. However, the shear wavepropagates much more slowly in tissue, approximately on the order of1-10 m/s.

Since the tissue motion is generally in the axial direction (i.e., theultrasound pulse direction) the shear waves may be detected usingconventional ultrasound Doppler techniques. In this regard, theultrasound Doppler technique is best suited to detect velocity in theaxial direction. Alternately, shear waves may be detected by measuring atissue displacement caused by the acoustic radiation force.

In order to accurately measure the propagation velocity of the shearwave, the shear wave needs to be tracked at a fast rate or a fast framerate of several thousands frames per second. An image in a frame mayconsist of a few hundred ultrasound lines. A typical frame rate ofregular ultrasound imaging is about 50 frames/s, which is too slow totrack the shear wave propagation. Therefore, there exists a need toincrease the frame rate while maintaining a good signal to noise ratioand good spatial resolution. Also, there exists a need to efficientlyprovide an indication of tissue stiffness.

SUMMARY

A method, medium and system may provide application of a firstultrasound pulse to biological tissue to create shear waves in thebiological tissue, transmission of a focused ultrasound pulse into thebiological tissue, reception of one or more ultrasound signals from thebiological tissue generated in response to the focused ultrasound pulse,detection of the shear waves in the biological tissue based on thereceived one or more ultrasound signals, determination of at least onepropagation property associated with the detected shear waves, anddisplay of the at least one propagation property associated with thedetected shear waves using a coding method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. A diagram of shear wave generation resulting from an acousticradiation force.

FIG. 2. A diagram of an ultrasound imaging system of some embodiments.

FIG. 3. A diagram of a conventional ultrasound imaging system.

FIG. 4. A diagram of multiple ultrasound transmitted/received beams.

FIG. 5. A diagram of an ultrasound transmitted beam and multipleultrasound received beams.

FIG. 6. Color coding of shear wave propagation velocity squared.

FIG. 7. Color coding of shear wave propagation velocity squared.

FIG. 8. A diagram illustrating generation of shear waves by acousticradiation forces and the propagation of shear waves.

FIG. 9. A diagram illustrating sliding movements of shear waves.

FIG. 10. A diagram illustrating the propagation of shear waves.

FIG. 11. A diagram illustrating the propagation of shear waves.

FIG. 12. An example of a color-coded image of shear wave propagationvelocity squared in tissue.

FIG. 13. A diagram to illustrate tissue displacement caused by anacoustic radiation force.

FIG. 14. Scale of shear wave velocity squared c² by color coding barcomposed of RGB representation.

FIG. 15. A diagram to show the ultrasound coordinate system with respectto an ultrasound transducer.

DETAILED DESCRIPTION

Embodiments will be described with reference to the accompanying drawingfigures wherein like numbers represent like elements throughout. Beforeembodiments of the invention are explained in detail, it is to beunderstood that embodiments are not limited in their application to thedetails of the examples set forth in the following description orillustrated in the figures. Other embodiments may be practiced orcarried out in a variety of applications and in various ways. Also, itis to be understood that the phraseology and terminology used herein isfor the purpose of description and should not be regarded as limiting.The use of “including,” “comprising,” or “having,” and variationsthereof herein is meant to encompass the items listed thereafter andequivalents thereof as well as additional items. The terms “mounted,”“connected,” and “coupled,” are used broadly and encompass both directand indirect mounting, connecting, and coupling. Further, “connected,”and “coupled” are not restricted to physical or mechanical connectionsor couplings.

Acoustic radiation force is created by a strong ultrasound pulse 120 asshown in FIG. 1. The ultrasound pulse 120 exhibits a high amplitude aswell as a long duration, (e.g., on the order of 100 microseconds). Theultrasound pulse 120 is transmitted from an ultrasound transducer array110. The ultrasound pulse 120 is focused at a focal point 130 inbiological tissue 160, resulting in an acoustic radiation force whichpushes the tissue 160 at the focal point 130. The ultrasound pulse 120may be transmitted multiple times and may be focused at a differentfocal point for each of multiple transmitted ultrasound pulses.

The tissue 160 is pushed mostly in the axial direction of the ultrasoundpulse 120, creating shear waves 140, 150 which may propagate in thelateral direction or directions other than the axial direction (i.e.,vertical direction). The propagation velocity of the shear waves 140,150 depends on the stiffness (Young's modulus or the shear modulus) ofthe tissue 160. Greater tissue stiffness results in greater shear wavepropagation velocity as shown in equation 1. Pathological conditionssuch as cancer may increase tissue stiffness thus these conditions maybe diagnosed by determining the propagation velocity. For example, theshear wave propagation velocity may vary from 1 m/s to 10 m/s, dependingon tissue conditions.

Since the shear wave may be characterized by tissue movement (ormotion), the shear wave may be detected by the ultrasound Dopplertechnique (e.g., see U.S. Pat. No. 4,573,477, U.S. Pat. No. 4,622,977,U.S. Pat. No. 4,641,668, U.S. Pat. No. 4,651,742, U.S. Pat. No.4,651,745, U.S. Pat. No. 4,759,375, U.S. Pat. No. 4,766,905, U.S. Pat.No. 4,768,515, U.S. Pat. No. 4,771,789, U.S. Pat. No. 4,780,837, U.S.Pat. No. 4,799,490, and U.S. Pat. No. 4,961,427). To detect this tissuemovement (motion), the ultrasound pulse is transmitted multiple times tothe tissue, and the ultrasound is scattered by scatterers in tissue andreceived by an ultrasound transducer as received ultrasound signals. Thereceived ultrasound signals from the ultrasound array transducers arefiltered, amplified, digitized, apotized, and beamformed (i.e. summed)after applying delays and/or phase-rotations for focusing and steering.The order of these processing steps may be interchanged. Receivedbeamformed RF ultrasound signals undergo quadrature demodulation,resulting in complex, Doppler I-Q signals. In a color Doppler technique,the ultrasound is transmitted at a pulse repetition frequency (PRF) andthe velocity is detected as the shift in frequency (Doppler shiftfrequency) in the received ultrasound signal. The received ultrasound ismixed with in-phase (0 degrees) and quadrature (90 degrees) referencesignals of the same frequency as the transmitted ultrasound frequency,resulting in complex I-Q Doppler signals.

Generally, the complex I-Q signal is used to derive the Doppler shiftfrequency because the Doppler shift frequency and the blood velocityhave the following relationship

$\begin{matrix}{{{\Delta \; f} = \frac{2f_{t}v\; \cos \; \theta}{c_{s}}},} & (2)\end{matrix}$

where Δf is the Doppler shift frequency, f_(t) is the transmittedfrequency, ν is the blood velocity, θ is the angle between theultrasound beam direction and the velocity vector, and c_(S) is thespeed of sound. The Doppler shift frequency is thus dependent on theangle between the velocity direction and the ultrasound beam directionand is a measurement that an ultrasound color Doppler system may obtain.

In the case of color Doppler, the number of the sampled signals may belimited to several. Therefore, an auto-correlation technique is usuallyused to determine the phase differences between the I-Q signals and thento determine the Doppler shift frequency and the velocity as follows.The color Doppler's I-Q signals z(m)=x(m)+jy(m) are used to calculate“auto-correlation” r as shown in the following equation, where z(m) isthe complex I-Q Doppler signal, x(m) is the in-phase (real) signal, y(m)is the quadrature phase (imaginary) signal, m indicates the signalnumber, j is the imaginary unit and * indicates the complex conjugate.

r=Σz(m)·z*(m−1)  (3)

The real (Re al(r)) and imaginary (Im ag(r)) parts of r are used toobtain the phase φ as shown in the following equation.

$\begin{matrix}{\phi = {\tan^{- 1}\frac{{Imag}(r)}{{Real}(r)}}} & (4)\end{matrix}$

Since tan⁻¹ usually provides only −0.5π to 0.5π, the position of complexvalue r in the complex coordinate may be also used to derive φ in therange of −π to π. The phase (i.e., color Doppler phase) φ is thenrelated to the Doppler shift frequency as shown in the followingequation.

$\begin{matrix}{{\Delta \; f} = \frac{\phi \; f_{PRF}}{2\pi}} & (5)\end{matrix}$

Autocorrelation r between the received complex baseband ultrasoundsignals is thus obtained to detect tissue velocity or movement.

Tissue movement is detected at multiple lateral points in a field oftissue region by multiple ultrasound beams (for example, 540, 545, 550in FIG. 5) in order to monitor movement. This movement reflects actionof the shear wave at those multiple lateral points (or multipleultrasound beams). Consequently, the lateral propagation velocity of theshear wave may be determined from the detected tissue movement.

Alternately, the shear wave may be detected by measuring tissuedisplacement caused by acoustic radiation force which is in turn causedby a strong ultrasound pulse as shown in FIG. 13. Tissue 1310 ispositioned at a position 1320 before the acoustic radiation is appliedand then is moved to a position 1330 after the acoustic radiation forcewas applied. To measure tissue displacement caused by the strongultrasound pulse, ultrasound pulses are transmitted to tissue from anultrasound transducer 1305 and then the ultrasound pulses are scatteredfrom scatterers in tissue and returned to the transducer 1305 andreceived by the transducer 1305 as received ultrasound signals. Theultrasound pulses are focused at a depth in order to increase asignal-to-noise ratio of the resulting received ultrasound signals incomparison to unfocused ultrasound pulses. Using correlation of thereceived ultrasound signals from tissue the displacement 1340 (from theposition 1320 to the position 1330) of the tissue 1310 due to theacoustic radiation force may be obtained and the tissue 1310 may betracked thereafter. The ultrasound pulses may thereby track shear wavesafter shear waves are created by acoustic radiation force.

Ultrasound signals resulting from the first ultrasound pulse andreceived from the tissue 1310 before acoustic radiation force is appliedare cross-correlated with received ultrasound signals resulting from thesecond ultrasound pulse after the acoustic radiation force is applied inorder to find the best match between the received ultrasound signals.The best match may be found by finding a maximum correlation value totrack the tissue and its displacement due to the acoustic radiationforce. Therefore, when tissue displacement is observed or measured, ashear wave is detected. The displacement and tissue velocity may berelated in that the displacement is a time integral ∫ν_(s)dt tissuevelocity ν_(s). Therefore, the tissue displacement may be obtained bycalculating the time integral of color Doppler velocity. Receivedultrasound signals may be RF (Radio Frequency), IF (IntermediateFrequency) or baseband signals after demodulation. Alternately, thedisplacement may be further differentiated to obtain tissue strain,which may be then used to detect the shear wave propagation velocity.

Cross correlation CC(t, τ) of signals in the previous paragraphs may bemathematically expressed as follows,

CC(t,τ)=∫_(t) ^(t+W) S ₁(t′)S ₂(t′−τ)dt′  (6)

where CC(t, τ): cross correlation; S₁(t′): received signal from thefirst ultrasound transmission; S₂(t′−τ): received ultrasound signal fromthe second ultrasound transmission; W: window length; t: time, t′: time;τ: time displacement. Time displacement value τ, which makes the maximumcross correlation (or the best match), determines the tissuedisplacement. Interpolation of signals using an interpolation function(e.g. cubic-spline) may be performed before cross correlation toincrease spatial resolution.

The cross correlation may be replaced by the sum of absolute differences(SAD), the sum of square differences (SSD), the sum of absolute cubicdifferences (SCD), or the sum of absolute power differences (SPD) asfollows.

$\begin{matrix}{{{SAD}\left\lbrack {l,k} \right\rbrack} = {\sum\limits_{n = 0}^{N}{{{S_{1}\left\lbrack {l + n} \right\rbrack} - {S_{2}\left\lbrack {l + n - k} \right\rbrack}}}}} & (7) \\{{{SSD}\left\lbrack {l,k} \right\rbrack} = {\sum\limits_{n = 0}^{N}\left( {{S_{1}\left\lbrack {l + n} \right\rbrack} - {S_{2}\left\lbrack {l + n - k} \right\rbrack}} \right)^{2}}} & (8) \\{{{SCD}\left\lbrack {l,k} \right\rbrack} = {\sum\limits_{n = 0}^{N}{{{S_{1}\left\lbrack {l + n} \right\rbrack} - {S_{2}\left\lbrack {l + n - k} \right\rbrack}}}^{3}}} & (9) \\{{{SPD}\left\lbrack {l,k} \right\rbrack} = {\sum\limits_{n = 0}^{N}{{{S_{1}\left\lbrack {l + n} \right\rbrack} - {S_{2}\left\lbrack {l + n - k} \right\rbrack}}}^{p}}} & (10)\end{matrix}$

S₁ is the received ultrasound signal from the first ultrasoundtransmission before displacement, S₂ is the received ultrasound signalfrom the second ultrasound transmission after displacement. N: thenumber of signals in the signal window. k: window displacement by thenumber of signals and equivalent of τ. l: the position of the window. pis a real number. For SAD, SSD, SCD and SPD, the tissue displacement isdetermined based on the value of k that makes the minimum (or bestmatch) of each of the SAD, SSD, SCD and SPD.

FIGS. 8 and 9 are used to illustrate shear wave generation and detectionin detail. A strong ultrasound pulse 820 is applied to tissue 860, 960from an ultrasound transducer 810, 910 once or more times to increasethe amplitude of shear waves which are caused by acoustic radiationforces resulting from the ultrasound pulse. Shear waves attenuate veryquickly in tissue and thus a greater amplitude results in a greaterpropagation distance. One or multiple ultrasound pulses may be focusedat one focal point or different focal points. The ultrasound pulsecreates acoustic radiation forces which push a layer of tissue,resulting in tissue movement 830, 910 mostly in the axial (vertical)direction as illustrated in FIG. 9. The tissue layer movement 910 causesadjacent tissue layer movements 920, 925 mostly in the axial direction.The tissue layer movements 920, 925 then in turn cause next tissue layermovements 930, 935 which then cause adjacent tissue layer movements 940,945. This succession of tissue movements represents a propagation ofshear waves 840, 850 in the lateral (horizontal) direction as shown inFIG. 8. Since the tissue movements (or motions) caused by acousticradiation forces are mostly in the axial direction, the motion may bedetected by the color Doppler technique, which is sensitive to motionsin the axial direction.

For example, the color Doppler technique transmits and receives severalultrasound pulses, determines phase differences between the receivedultrasound signals, and calculates a velocity of tissue or blood usingthe autocorrelation technique as previously discussed and known in theart. Variance and power of color Doppler signals may be also calculatedin addition to the velocity. As in the conventional display of movingtissue or blood, one of these parameters may be used to display shearwaves as shown in FIGS. 10, 11. It will be assumed that shear waves 1040(1140), 1050 (1150) are determined in a color Doppler frame representinga certain time and shear waves 1060 (1160), 1070 (1170) are determinedat a next moment or in a next frame. More image frames of shear wavesmay be obtained to track the shear waves and to create a movie of shearwave propagation. In alternate embodiments, tissue displacement due toacoustic radiation forces may be detected.

FIGS. 10 and 11 depict shear wave propagation at two points in time.Local shear wave propagation velocities, as illustrated by arrows 1080,1090, may be derived by correlating two images of shear waves at twopoints in time. More image frames of shear waves may be used to trackthe propagation of shear waves in more image areas in order to presentlocal shear wave propagation velocities or shear wave propagationvelocity squared in a two-dimensional image as described below.

Correlation coefficient (CCV) between a first frame signal S¹ and thesecond frame signal S² may be obtained as speckle tracking as follows,

$\begin{matrix}{{{CCV}\left( {S^{1},S^{2}} \right)} = \frac{\sum\limits_{x = 1}^{m}{\sum\limits_{z = 1}^{n}{\left( {S_{x,z}^{1} - \overset{\_}{S^{1}}} \right)\left( {S_{{x + X},{z + Z}}^{2} - \overset{\_}{S^{2}}} \right)}}}{\sqrt{\sum\limits_{x = 1}^{m}{\sum\limits_{z = 1}^{n}{\left( {S_{x,z}^{1} - \overset{\_}{S^{1}}} \right)^{2} \cdot {\sum\limits_{x = 1}^{m}{\sum\limits_{z = 1}^{n}\left( {S_{{x + X},{z + Z}}^{2} - \overset{\_}{S^{2}}} \right)^{2}}}}}}}} & (11)\end{matrix}$

where S¹ _(x,z) is the ultrasound signal at x, z of the first frame, S²_(x+X,z+Z) is the ultrasound signal at x+X, z+Z of the second frame, S¹is mean signal value in the window of the first frame signal, S² is meansignal value in the window of the second frame signal. The coordinatesystem (x,y,z) is shown with respect to an ultrasound transducer 1510 inFIG. 15. The elevational axis y is perpendicular to the paper of FIG. 15although it is shown slightly different for illustration purposes.

The displacement X, Z, that yields the maximum correlation coefficientdetermines the correct speckle tracking and the distance, and thus thevelocity (i.e., the distance per time).

Similar to the 1D case, the correlation coefficient may be replaced bythe sum of absolute differences (SAD), the sum of square differences(SSD), the sum of absolute cubic differences (SCD) and the sum ofabsolute power differences (SPD) as follows.

$\begin{matrix}{{{SAD}\left( {S^{1},S^{2},X,Z} \right)} = {\sum\limits_{x = 1}^{m}{\sum\limits_{z = 1}^{n}{{S_{x,z}^{1} - S_{{x + X},{z + Z}}^{2}}}}}} & (12) \\{{{SSD}\left( {S^{1},S^{2},X,Z} \right)} = {\sum\limits_{x = 1}^{m}{\sum\limits_{z = 1}^{n}\left( {S_{x,z}^{1} - S_{{x + X},{z + Z}}^{2}} \right)^{2}}}} & (13) \\{{{SCD}\left( {S^{1},S^{2},X,Z} \right)} = {\sum\limits_{x = 1}^{m}{\sum\limits_{z = 1}^{n}{{S_{x,z}^{1} - S_{{x + X},{z + Z}}^{2}}}^{3}}}} & (14) \\{{{SPD}\left( {S^{1},S^{2},X,Z} \right)} = {\sum\limits_{x = 1}^{m}{\sum\limits_{z = 1}^{n}{{S_{x,z}^{1} - S_{{x + X},{z + Z}}^{2}}}^{p}}}} & (15)\end{matrix}$

p is a real number; m and n are integers. The 2D speckle tracking may beapproximated by a 1D speckle tracking to obtain the shear wavepropagation velocity and the shear wave propagation velocity squared.The mathematical expression will be similar to that used in thedisplacement measurement.

Alternately, a shear wave equation (16) may be used to derive the shearwave propagation velocity as follows,

$\begin{matrix}{{\rho \frac{\partial^{2}u_{i}}{\partial t^{2}}} = {\mu\left( {\frac{\partial^{2}u_{i}}{\partial x^{2}} + \frac{\partial^{2}u_{i}}{\partial y^{2}} + \frac{\partial^{2}u_{i}}{\partial z^{2}}} \right)}} & (16)\end{matrix}$

where i=x, y, z, ρ is tissue density, μ is the shear modulus, u_(i) isthe displacement vector, x is lateral coordinate, y is elevationalcoordinate and z is axial coordinate as shown in FIG. 15. Forincompressible materials, the Young's modulus E and the shear modulus μhave the following relationship.

E=3μ  (17)

Therefore, the shear wave propagation velocity squared may be obtainedas a ratio of the shear modulus to the density as the followingequation.

$\begin{matrix}{c^{2} = \frac{\mu}{\rho}} & (18)\end{matrix}$

One of the displacement components u_(z) in equation 16 may bedetermined by cross-correlation as previously discussed. By combining zcomponent of equation 16 and equation 18, the shear wave propagationvelocity squared and velocity are obtained as follows,

$\begin{matrix}{c^{2} = \frac{\frac{\partial^{2}u_{z}}{\partial t^{2}}}{\frac{\partial^{2}u_{z}}{\partial x^{2}} + \frac{\partial^{2}u_{z}}{\partial y^{2}} + \frac{\partial^{2}u_{z}}{\partial z^{2}}}} & (19) \\{and} & \; \\{c = {\sqrt{\frac{\frac{\partial^{2}u_{z}}{\partial t^{2}}}{\frac{\partial^{2}u_{z}}{\partial x^{2}} + \frac{\partial^{2}u_{z}}{\partial y^{2}} + \frac{\partial^{2}u_{z}}{\partial z^{2}}}}.}} & (20)\end{matrix}$

Therefore, the shear wave propagation velocity is obtained as the squareroot of the ratio between the temporal second-order derivative of thedisplacement and the spatial second-order derivatives of thedisplacement. Likewise, the shear wave propagation velocity squared isobtained as the ratio between the temporal second-order derivative ofthe displacement and the spatial second-order derivatives of thedisplacement. Since the spatial derivative of the displacement inelevational direction

$\frac{\partial^{2}u_{z}}{\partial y^{2}}$

may be considered negligible compared with the other spatialderivatives, the shear wave propagation velocity squared and velocitymay be obtained from the other measurement values.

It is desirable to monitor and to track the shear wave frequently,meaning at a fast rate or frame rate. To speed up the frame rate, awide, focused ultrasound pulse 520 may be transmitted and multipleultrasound signals 540, 545, 550 may be simultaneously received as shownin FIG. 5. The received ultrasound beams are used as describedpreviously to detect shear waves and to derive shear wave propagationproperties (i.e., velocity and velocity squared) therefrom. The focusedtransmit ultrasound beam 520 may be particularly suitable formaintaining a good signal-to-noise ratio of resulting receivedultrasound beams during the detection of shear waves.

In some embodiments, multiple ultrasound beams (pulses) aresimultaneously applied and transmitted to the tissue field and multipleultrasound beams (pulses) per transmitted ultrasound pulse are receivedto increase the frame rate, as shown in FIG. 4. In FIG. 4, ultrasoundpulses 420, 430 are simultaneously transmitted to biological tissue 480from an ultrasound transducer array 410. For each transmitted ultrasoundpulse 420, 430, multiple ultrasound receive signals 440, 445, 465, 460,465, 470 are simultaneously received. The multiple ultrasound pulses maybe transmitted simultaneously or at substantially simultaneous times.The multiple ultrasound pulses may be simultaneously transmitted. Or asecond ultrasound pulse may be transmitted after a first ultrasoundpulse is transmitted and before the first ultrasound pulse returns tothe ultrasound transducer from a deepest depth of an ultrasound field.This transmission method increases the frame rate.

FIG. 4 shows an example of two simultaneous transmitted ultrasoundpulses but more than two transmitted ultrasound pulses may be also used.In some embodiments, coded ultrasound waveforms may be transmitted forbetter separation of simultaneous multiple ultrasound signals. Forexample, chirp codes, Barker codes, Golay codes or Hadamard codes may beused for better separation of ultrasound pulses. Again, the receivedsignals are analyzed using the methods previously described to determinetissue movement at multiple points, and shear wave propagationproperties are derived therefrom.

An image of a shear wave can be created based on the motion (orvelocity) detected at multiple points in the imaging field. Subsequenttransmit/receive sequences of ultrasound may create multiple images ofthe shear wave at multiple points in time. Correlation between theimages of the shear wave is then calculated to obtain the shear wavepropagation velocity and velocity squared as previously discussed.Alternately, tissue displacement caused by acoustic radiation force isdetermined and the shear wave propagation velocity is calculated as thesquare root of the ratio between the temporal second-order derivative ofthe displacement and the spatial second-order derivatives of thedisplacement. Likewise, the shear wave propagation velocity squared iscalculated as the ratio between the temporal second-order derivative ofthe displacement and the spatial second-order derivatives of thedisplacement.

In some embodiments, the propagation velocity of a detected shear wave(c) may be displayed. In some embodiments, the propagation velocitysquared (c²) of the detected shear wave may be displayed.Advantageously, the propagation velocity squared (c²) may be moreclosely related than the propagation velocity (c) to the Young's modulusor the shear modulus as shown in equation 1. Therefore the propagationvelocity squared (c²) may provide an efficient proxy for the actualstiffness. In some embodiments, the propagation velocity squared (c²)may be multiplied by three and then displayed. If tissue density isclose to 1 g/cm³, this number (i.e., 3c²) may be close to the actualYoung's modulus. In some embodiments, a product (bc²) of any real number(b) and the propagation velocity squared (c²) may be displayed.Determinations of actual stiffness are difficult and error-prone becausethe density of the tissue is unknown and must be estimated.

A color coding technique, a grayscale technique, or a graphical codingtechnique may be employed to present a shear wave propagation property(i.e., velocity c or velocity squared c²) to a user. In someembodiments, a propagation velocity squared (c²) of shear waves withintissue is displayed in a two-dimensional color image. Graphical-codingand/or two-dimensional images may also be used to represent thepropagation velocity c or velocity squared c² in some embodiments.

A low value of shear wave propagation velocity squared c² may be codedusing a red color while a high value of c² may be coded using a bluecolor. For example, FIG. 6 illustrates a legend indicating that ared-colored tissue area includes shear waves associated with low c²values (e.g., 1 m²/s²) and that a blue-colored tissue area includesshear waves associated with high c² values (e.g., 100 m²/s²).Embodiments are not limited to color-based coding. Images of shear wavepropagation properties within tissue may be coded using grayscale or anycombination of graphical patterns (e.g., vertical lines, horizontallines, cross-hatching, dot patterns of different densities, etc.) andcolors.

After determining the propagation velocity squared (c²), c² may be codedlinearly with respect to the color wavelength as shown in FIG. 6. Forexample, if c² within a tissue area is determined to be 50 m²/s², thetissue area may be displayed using a yellow color 630.

Alternately, color-coding of the shear wave propagation velocity squared(c²) may be defined as shown in FIG. 7. Tissue areas associated with lowvalues of the shear wave propagation velocity squared may be displayedas blue 710 while areas associated with high values of the velocitysquared may be displayed as red 720. Different color-coding methods maybe also used to represent the propagation velocity squared (c²) orvelocity c of shear waves. For example, color coding may be based onhue, brightness, and other color characteristics. The color-coded scalemay represent different maximums and minimums of the shear wavepropagation velocity squared or velocity than shown in FIG. 6, 7. Inthis regard, the velocity squared maximum of 100 m²/s² and velocitysquared minimum of 1 m²/s² in FIGS. 6 and 7 are only for theillustration purposes and do not limit the scope of the claims. Othervalues may represent the maximum or minimum values of the coding scale.

Color coding based on Red, Green and Blue (RGB) values may be used torepresent the propagation velocity c or velocity squared (c²) of shearwaves as shown in FIG. 14. In this example (FIG. 14), the propagationvelocity squared (c²) of a shear wave within tissue is representedaccording to a color coding bar 1410 which is based on RGB values 1420,1430 and 1440. The shear wave propagation velocity squared has 256possible values in this example, as represented 256 colors in the colorcoding bar 1410. The smallest velocity squared c²(0) 1412 is representedby a color composed of a combination of R(0) 1422, G(0) 1432 and B(0)1442. The middle velocity squared c²(127) 1415 is represented by a colorcomposed of a combination of R(127) 1425, G(127) 1435 and B(127) 1445.The highest velocity squared c²(255) 1418 is represented by a colorcomposed of a combination of R(255) 1428, G(255) 1438 and B(255) 1448.In this example, R(255) only indicates a Red color associated with thered index 255 and does not necessarily indicate a Red color value of255, which is the brightest Red color. Likewise, G(255) indicates aGreen color associated with the green index 255 and B(255) indicates aBlue color associated with the blue index 255.

Alternately, Red, Green, Blue and Yellow may be used to define a colorcoding bar. Alternately, a Hue-based color coding bar may be used.

FIG. 12 represents an example of a color-coded image 1260 displaying ashear wave propagation velocity squared c² within human soft tissue(e.g. breast). A color coding scale 1250 is illustrated, in which acolor code 1210 (i.e., representing a red color although displayed aswhite in this black/white document) represents a low shear wavepropagation velocity squared value and a color code 1220 (i.e.,representing a blue color although displayed as hatched in thisblack/white document) represents a higher shear wave propagationvelocity squared value.

Based on the coding scale 1250, it can be seen that the color codedimage 1260 includes an area 1280 of high propagation velocity squaredc². Since the shear wave propagation velocity squared c² is proportionalto the Young's modulus, the tissue area corresponding to area 1280 islikely to be hard. Since a tumor is generally hard, image 1260 mayindicate pathological conditions.

The color-coding method provides efficient distinction between an areaincluding shear waves having a high propagation velocity squared valueand other areas including shear waves having a low propagation velocitysquared value. The color coding method therefore allows efficientidentification of hard tissue areas within soft tissue areas. An imagedisplaying shear wave propagation velocity or velocity squared may becombined (e.g., superimposed) with a regular image of ultrasound, e.g.B-mode image, or a combined B-mode image and color Doppler image and/orspectral Doppler image. Alternately, the shear wave propagation velocitysquared or velocity may be displayed numerically. In some embodiments,the shear wave propagation velocity squared may be displayed in grayscale or based on other graphic coding methods such as using patternsrather than colors. For example, low values of shear wave propagationvelocity or square of the shear wave propagation velocity may bedisplayed in black or dark gray while high values of shear wavepropagation velocity or shear wave propagation velocity squared may bedisplayed in light gray or white using a grayscale coding method.

FIG. 3 shows a diagram of a conventional ultrasound diagnostic imagingsystem with B-mode imaging, Doppler spectrum and color Doppler imaging.The system may include other imaging modes, e.g. elasticity imaging, 3Dimaging, real-time 3D imaging, tissue Doppler imaging, tissue harmonicimaging, contrast imaging and others. An ultrasound signal istransmitted from an ultrasound probe 330 driven by atransmitter/transmit beamformer 310 through a transmit/receive switch320. The probe 320 may consist of an array of ultrasound transducerelements which are separately driven by the transmitter/transmitbeamformer 310 with different time-delays so that a transmit ultrasoundbeam is focused and steered. A receive beamformer 340 receives thereceived ultrasound signals from the probe 330 through the switch 320and processes the signals 325. The receive beamformer 340 applies delaysand/or phases to the signals and the resultant signals are summed forfocusing and steering a received ultrasound beam. The receive beamformer340 may also apply apodization, amplification and filtering.

The processed signal 345 is coupled to a Doppler spectrum processor 350,a color Doppler processor 360, and a B-mode image processor 370. TheDoppler spectrum processor 350 includes a Doppler signal processor and aspectrum analyzer, and processes Doppler flow velocity signals andcalculates and outputs a Doppler spectrum 355. The color Dopplerprocessor 360 processes the received signal 345 and calculates andoutputs velocity, power and variance signals 365. The B-mode imageprocessor 370 processes the received signal 345 and calculates andoutputs a B-mode image 375 or the amplitude of the signal by anamplitude detection.

The Doppler spectrum signals 355, color Doppler processor signals(velocity, power, and variance) 365 and B-mode processor signals 375 arecoupled to a scan converter 380 that converts the signals toscan-converted signals. The output of scan converter 380 is coupled to adisplay monitor 390 for displaying ultrasound images.

FIG. 2 shows a diagram of elements of an ultrasound imaging systemincluding a shear wave processor 295 according to some embodiments. Theultrasound system in FIG. 2 transmits strong ultrasound pulses tobiological tissue to create acoustic radiation forces which push thebiological tissue. Shear waves are created and propagate in the tissueafter the biological tissue is pushed. The ultrasound system thentransmits and receives ultrasound pulses to track the shear waves as theshear waves propagate in the biological tissue. Multiple receivedultrasound beams may be simultaneously formed by the receive beamformer240. Likewise, multiple transmitted ultrasound beams may besimultaneously formed by the transmitter/transmit beamformer 210.Received ultrasound signals from the receive beamformer 240 areprocessed to obtain tissue displacement, Doppler velocity, correlation,shear wave propagation velocity and/or shear wave propagation velocitysquared as previously described. The shear wave processor 295 mayperform the shear wave processing methods described previously. Theshear wave processor 295 receives output 245 from the receive beamformer240. Output 297 comprises shear wave velocity data or other shear waveproperties. For example, the shear wave processor 295 outputs thepropagation velocity or the square of the propagation velocity of theshear wave to a scan converter 280 and a representation of the shearwave propagation velocity or the square of the shear wave propagationvelocity is output to the display monitor along with the B-mode, colorDoppler or spectral Doppler images.

The shear wave processor 295 may comprise of general purpose centralprocessing units (CPUs), digital signal processors (DSPs), fieldprogrammable Arrays (FPGAs), graphic processing units (GPUs) and/ordiscreet electronics devices.

FIG. 2 represents a logical architecture according to some embodiments,and actual implementations may include more or different elementsarranged in other manners. Other topologies may be used in conjunctionwith other embodiments. Moreover, each element of the FIG. 2 system maybe implemented by any number of computing devices in communication withone another via any number of other public and/or private networks. Twoor more of such computing devices may be located remote from one anotherand may communicate with one another via any known manner of network(s)and/or a dedicated connection. The system may comprise any number ofhardware and/or software elements suitable to provide the functionsdescribed herein as well as any other functions. For example, anycomputing device used in an implementation of the FIG. 2 system mayinclude a processor to execute program code such that the computingdevice operates as described herein.

All systems and processes discussed herein may be embodied in programcode stored on one or more non-transitory computer-readable media. Suchmedia may include, for example, a floppy disk, a CD-ROM, a DVD-ROM, aBlu-ray disk, a Flash drive, magnetic tape, and solid state RandomAccess Memory (RAM) or Read Only Memory (ROM) storage units. Embodimentsare therefore not limited to any specific combination of hardware andsoftware.

One or more embodiments have been described. Nevertheless, variousmodifications will be apparent to those in the art.

1. A method comprising: applying a first ultrasound pulse to biologicaltissue to create shear waves in the biological tissue; transmitting afocused ultrasound pulse into the biological tissue; receiving one ormore ultrasound signals from the biological tissue generated in responseto the focused ultrasound pulse; detecting the shear waves in thebiological tissue based on the received one or more ultrasound signals;determining at least one propagation property associated with thedetected shear waves; and displaying the at least one propagationproperty associated with the detected shear waves using a coding method.2. A method according to claim 1, wherein the at least one shear wavepropagation property comprises one or more of; a propagation velocityassociated with one or more of the detected shear waves; and a product(bc²) of a real number (b) and the square of the shear wave propagationvelocity (c²).
 3. A method according to claim 1, wherein detecting theshear waves comprises calculating a correlation, the sum of absolutedifferences (SAD), the sum of square differences (SSD), the sum ofabsolute cubic differences (SCD) or the sum of absolute powerdifferences (SPD) between the received ultrasound signals at one ormultiple positions in time.
 4. A method according to claim 1, whereindetermining the at least one propagation property comprises calculatinga correlation, the sum of absolute differences (SAD), the sum of squaredifferences (SSD), the sum of absolute cubic differences (SCD) or thesum of absolute power differences (SPD) between the detected shear wavesat one or multiple instances.
 5. A method according to claim 1, furthercomprising transmitting a second focused ultrasound pulse from atransducer after transmitting a first focused ultrasound pulse andbefore the first focused ultrasound pulse returns to the transducer froma deepest position in an ultrasound field.
 6. A method according toclaim 1, wherein the transmitted focused ultrasound pulses comprisecoded waveform signals.
 7. A method according to claim 6, wherein thecoded waveform signals comprise one of Chirp codes, Barker codes, Golaycodes or Hadamard codes.
 8. A method according to claim 1, wherein thecoding method comprises color coding, grayscale coding or a numericaldisplay.
 9. A method according to claim 8, wherein the color coding isbased on RGB (Red, Green, Blue) values, RGBY (Red, Green, Blue, Yellow)values, Hue, luminance, the wavelength or a color chart.
 10. A methodaccording to claim 1, wherein detecting the shear waves comprisesdetermining a displacement of the biological tissue.
 11. A methodaccording to claim 1, wherein detecting the shear waves comprisesdetermining a velocity of the biological tissue using color Dopplertechnique.
 12. A method according to claim 2, wherein the shear wavepropagation velocity is calculated based on the square root of the ratiobetween a temporal second-order derivative of the displacement of thebiological tissue and a spatial second-order derivative of thedisplacement of the biological tissue.
 13. A method according to claim2, wherein the square of the shear wave propagation velocity iscalculated based on the ratio between a temporal second-order derivativeof the displacement of the biological tissue and a spatial second-orderderivative of the displacement of the biological tissue.
 14. A methodaccording to claim 10, wherein determining a displacement of thebiological tissue comprises calculating a time integral of tissue colorDoppler velocity.
 15. A method according to claim 1, wherein applyingthe first ultrasound pulse comprises applying a plurality of ultrasoundpulses to the biological tissue to create shear waves in the biologicaltissue, wherein each of the plurality of ultrasound pulses is focused ata different focal point.
 16. A method according to claim 1, whereintransmitting the focused ultrasound pulse comprises transmitting aplurality of focused ultrasound pulses into the biological tissue morethan one time, and wherein the one or more ultrasound signals arereceived from the biological tissue at one or more instances.
 17. Anon-transitory medium storing processor-executable program code, theprogram code executable by a device to: apply a first ultrasound pulseto biological tissue to create shear waves in the biological tissue;transmit a focused ultrasound pulse into the biological tissue; receiveone or more ultrasound signals from the biological tissue generated inresponse to the focused ultrasound pulse; detect the shear waves in thebiological tissue based on the received one or more ultrasound signals;determine at least one propagation property associated with the detectedshear waves; and display the at least one propagation propertyassociated with the detected shear waves using a coding medium.
 18. Amedium according to claim 17, wherein the at least one shear wavepropagation property comprises one or more of; a propagation velocityassociated with one or more of the detected shear waves; and a product(bc²) of a real number (b) and the square of the shear wave propagationvelocity (c²).
 19. A medium according to claim 17, wherein detection ofthe shear waves comprises calculation of a correlation, the sum ofabsolute differences (SAD), the sum of square differences (SSD), the sumof absolute cubic differences (SCD) or the sum of absolute powerdifferences (SPD) between the received ultrasound signals at one ormultiple positions in time.
 20. A medium according to claim 17, whereindetermination of the at least one propagation property comprisescalculation of a correlation, the sum of absolute differences (SAD), thesum of square differences (SSD), the sum of absolute cubic differences(SCD) or the sum of absolute power differences (SPD) between thedetected shear waves at one or multiple instances.
 21. A mediumaccording to claim 17, further comprising transmission of a secondfocused ultrasound pulse from a transducer after transmitting a firstfocused ultrasound pulse and before the first focused ultrasound pulsereturns to the transducer from a deepest position in an ultrasoundfield.
 22. A medium according to claim 17, wherein the transmittedfocused ultrasound pulses comprise coded waveform signals.
 23. A mediumaccording to claim 22, wherein the coded waveform signals comprise oneof Chirp codes, Barker codes, Golay codes or Hadamard codes.
 24. Amedium according to claim 17, wherein the coding medium comprises colorcoding, grayscale coding or a numerical display.
 25. A medium accordingto claim 24, wherein the color coding is based on RGB (Red, Green, Blue)values, RGBY (Red, Green, Blue, Yellow) values, Hue, luminance, thewavelength or a color chart.
 26. A medium according to claim 17, whereindetection of the shear waves comprises determination of a displacementof the biological tissue.
 27. A medium according to claim 17, whereindetection of the shear waves comprises determination of a velocity ofthe biological tissue using color Doppler technique.
 28. A mediumaccording to claim 18, wherein the shear wave propagation velocity iscalculated based on the square root of the ratio between a temporalsecond-order derivative of the displacement of the biological tissue anda spatial second-order derivative of the displacement of the biologicaltissue.
 29. A medium according to claim 18, wherein the square of theshear wave propagation velocity is calculated based on the ratio betweena temporal second-order derivative of the displacement of the biologicaltissue and a spatial second-order derivative of the displacement of thebiological tissue.
 30. A medium according to claim 26, whereindetermination of a displacement of the biological tissue comprisescalculation of a time integral of tissue color Doppler velocity.
 31. Amedium according to claim 17, wherein application of the firstultrasound pulse comprises application of a plurality of ultrasoundpulses to the biological tissue to create shear waves in the biologicaltissue, wherein each of the plurality of ultrasound pulses is focused ata different focal point.
 32. A medium according to claim 17, whereintransmission of the focused ultrasound pulse comprises transmission of aplurality of focused ultrasound pulses into the biological tissue morethan one time, and wherein the one or more ultrasound signals arereceived from the biological tissue at one or more instances.
 33. Asystem comprising: a memory storing processor-executable program code;and a processor to execute the processor-executable program code inorder to cause the system to: apply a first ultrasound pulse tobiological tissue to create shear waves in the biological tissue;transmit a focused ultrasound pulse into the biological tissue; receiveone or more ultrasound signals from the biological tissue generated inresponse to the focused ultrasound pulse; detect the shear waves in thebiological tissue based on the received one or more ultrasound signals;determine at least one propagation property associated with the detectedshear waves; and display the at least one propagation propertyassociated with the detected shear waves using a coding system.
 34. Asystem according to claim 33, wherein the at least one shear wavepropagation property comprises one or more of; a propagation velocityassociated with one or more of the detected shear waves; and a product(bc²) of a real number (b) and the square of the shear wave propagationvelocity (c²).
 35. A system according to claim 33, wherein detection ofthe shear waves comprises calculation of a correlation, the sum ofabsolute differences (SAD), the sum of square differences (SSD), the sumof absolute cubic differences (SCD) or the sum of absolute powerdifferences (SPD) between the received ultrasound signals at one ormultiple positions in time.
 36. A system according to claim 33, whereindetermination of the at least one propagation property comprisescalculation of a correlation, the sum of absolute differences (SAD), thesum of square differences (SSD), the sum of absolute cubic differences(SCD) or the sum of absolute power differences (SPD) between thedetected shear waves at one or multiple instances.
 37. A systemaccording to claim 33, the processor further to execute theprocessor-executable program code in order to cause the system to:transmit a second focused ultrasound pulse from a transducer aftertransmitting a first focused ultrasound pulse and before the firstfocused ultrasound pulse returns to the transducer from a deepestposition in an ultrasound field.
 38. A system according to claim 33,wherein the transmitted focused ultrasound pulses comprise codedwaveform signals.
 39. A system according to claim 38, wherein the codedwaveform signals comprise one of Chirp codes, Barker codes, Golay codesor Hadamard codes.
 40. A system according to claim 33, wherein thecoding system comprises color coding, grayscale coding or a numericaldisplay.
 41. A system according to claim 40, wherein the color coding isbased on RGB (Red, Green, Blue) values, RGBY (Red, Green, Blue, Yellow)values, Hue, luminance, the wavelength or a color chart.
 42. A systemaccording to claim 33, wherein detection of the shear waves comprisesdetermination of a displacement of the biological tissue.
 43. A systemaccording to claim 33, wherein detection of the shear waves comprisesdetermination of a velocity of the biological tissue using color Dopplertechnique.
 44. A system according to claim 34, wherein the shear wavepropagation velocity is calculated based on the square root of the ratiobetween a temporal second-order derivative of the displacement of thebiological tissue and a spatial second-order derivative of thedisplacement of the biological tissue.
 45. A system according to claim34, wherein the square of the shear wave propagation velocity iscalculated based on the ratio between a temporal second-order derivativeof the displacement of the biological tissue and a spatial second-orderderivative of the displacement of the biological tissue.
 46. A systemaccording to claim 42, wherein determination of a displacement of thebiological tissue comprises calculation of a time integral of tissuecolor Doppler velocity.
 47. A system according to claim 33, whereinapplication of the first ultrasound pulse comprises application of aplurality of ultrasound pulses to the biological tissue to create shearwaves in the biological tissue, wherein each of the plurality ofultrasound pulses is focused at a different focal point.
 48. A systemaccording to claim 33, wherein transmission of the focused ultrasoundpulse comprises transmission of a plurality of focused ultrasound pulsesinto the biological tissue more than one time, and wherein the one ormore ultrasound signals are received from the biological tissue at oneor more instances.